Euclid's Elements. Book 1. Proposition 7.
Given two straight lines constructed from the ends of a straight line and meeting in a point, there cannot be constructed from the ends of the same straight line, and on the same side of it, two other straight lines meeting in another point and equal to the former two respectively, namely each equal to that from the same end. This proofs why and how two straight lines cannot be constructed upon another straight line and be equal to two other straight lines that were already constructed upon it on the same side.

Video Instructed by: Pablito Velasquez.
